Thermoplastic resin blends are increasingly common in the plastics industry today. These thermoplastic resin compositions may be produced from two or more thermoplastic resins or from thermoplastic resin(s) and various other components, including crosslinked elastomers, fillers, plasticizers, and the like. While one or more of the components above may not themselves be processible using conventional thermoplastic molding equipment, e.g. crosslinked elastomers or mineral fillers, the resultant blends are designed to be processed using one of several types of thermoplastic forming equipment, for example injection molding machines and blow molding machines.
One of the earliest types of blends developed was a filled blend, i.e. a thermoplastic resin which is filled with an inert filler to produce a reinforced thermoplastic blend. The filler may be particulate, fibrous or of some other form. Generally, a filled blend contains a relatively large amount of the filler. Preparation of filled blends involves uniformly distributing the filler particles throughout the thermoplastic, and maintaining the thermoplastic resin as a continuous phase (matrix), which allows melt fabrication. Melt fabrication may be prevented if high levels of filler or interaction between filler particles reduce the thermoplastic nature of the melt.
Another common type of blend is a toughened thermoplastic. Ideally, a rubber-like material is distributed in small particles throughout a matrix thermoplastic in the blend, although often complex phase structures can exist. The particle size of the rubber-like material or toughener is frequently important in achieving maximum toughness. In contrast to filled thermoplastic blends where the already formed filler particles are merely distributed throughout the matrix, the production of toughened thermoplastics blends will generally involve breakdown of large particles of the rubber-like toughener into smaller ones, in addition to distribution of the toughener particles throughout the matrix resin. Thus, the toughener is dispersed as well as distributed. In addition, the dispersing of the toughener may involve a form of mixing which involves high shear. In toughened thermoplastic blends, the thermoplastic resin is usually more than 70 percent of the blend.
Processing and fabrication of such thermoplastic resin blends has heretofore been conducted in two discrete steps: first, melt blending of the components of the blend (e.g. in an extruder) and formation of the blend into cooled resin pellets, and second, remelting the resin pellets, shaping the blend in a thermoplastic molding device, (e.g. in an injection molding or blow molding machine), and finally re-cooling the shaped article made from the resin blend.
Recently a new type of thermoplastic blend has been developed which will hereinafter be referred to as a "partially grafted flexible thermoplastic composition". These compositions are disclosed in U.S. Pat. No. 4,871,810, granted Oct. 3, 1989 to Saltman (hereinafter referred to as Saltman I), and in PCT Patent Publication WO 88/03543, published May 19, 1988 (hereinafter referred to as Saltman II). Both references are hereby incorporated by reference. These compositions contain at least three polymeric components, namely, a thermoplastic resin, component (a), which may be non-reactive (Saltman I) or reactive (Saltman II); an ethylene acid copolymer or its derived ionomer, component (b); and a polymeric grafting agent, component (c).
Saltman I and II describe the melt processing and fabrication of these compositions in two discrete steps as generally described above, except that Saltman emphasizes the importance in the first step (forming the melt blended pellets) of conditions that will enable chemical reaction between at least components (b) and (c). In the Examples in Saltman I and II, compositions are pre-blended into pellets by dry mixing the ingredients in a "salt and pepper" blend, followed by extrusion in a 28 mm twin screw extruder using a high shear screw. Saltman I, also uses a System 40 Haake rheocord, using a Banbury attachment for pre-blending some of the compositions. Other conventional plasticating devices such as a Brabender or Banbury mill are disclosed for pre-blending to yield molding pellets. The molding pellets are formed into shaped articles in a discrete fabrication step, which starts with feeding of the molding pellets to an injection molding machine.
Thus in both Saltman I and Saltman II, the shaped articles formed from the partially grafted flexible thermoplastic resin compositions were produced in two steps. Specifically, the thermoplastic resins were first compounded to form molding pellets (hereinafter referred to as pre-compounding), followed by a second step of injection molding to form the shaped articles. The chemical reaction is accomplished in the pre-compounding step; the fabrication is accomplished in the injection molding step.
Both component (b) and component (c) are thermoplastic and melt processible by themselves. In the pre-compounding step disclosed in Saltman I and II, components (b) and (c), however, react with each other during the shearing of the molten composition which results in a single partially grafted phase. Component (c) contains a reactive group, typically a glycidyl group which reacts with the acid group in component (b); and (i) reacts with the thermoplastic resin, component (a), if the thermoplastic has reactive groups (Saltman II) or (ii) is compatible with component (a) (Saltman I).
The thermoplastic resin, component (a), is present in Saltman I and Saltman II compositions in no greater than 50 volume percent of the total of components (a) plus (b) plus (c), yet component (a) becomes at least one continuous phase. It is believed that this phase morphology results from the reaction at least between components (b) and (c) during the pre-compounding step and enables the composition to be flexible, thermoplastically processible and to achieve good mechanical properties at high temperatures.
Component (c), the polymeric grafting agent, achieves a highly controllable and reproducible level of grafting in the compositions of Saltman I and II. The polymeric grafting agent has a carefully defined level of reactive groups in relation to the level of acid groups in the ethylene acid copolymer, component (b). If the reactive group relationship is not satisfied, the desired proper morphology and therefore the desired properties are not achieved. If, too little grafting is achieved, the thermoplastic resin does not become a continuous phase as required; or if too much grafting occurs, the blend approaches intractibility.
Thermoplastic resin blends can comprise a wide range of types, with a wide range of components, proportions and properties. Accordingly, it is not surprising that blending means have to be found to best produce each particular type. For example, incorporating a colorant may involve only distributive mixing, where color particles are uniformly distributed in the matrix resin; and fiber filled resins may also require distributive mixing, but there may be critical conditions required not to break the fibers, so that high shear conditions may be unacceptable. However, incorporating a toughener may involve both distributive and dispersive mixing. The latter is generally a higher shear process. In the case of the partially grafted flexible thermoplastic resin compositions of Saltman I and II, at least components (b) and (c) are required to react with each other chemically as well as be intimately blended and further achieve a proper morphology, namely, wherein component (a), which is not the major volume component, is at least one continuous phase of the melt processed composition. Proper mixing to accomplish all of these objectives has heretofore only been achieved using a discrete pre-compounding step.
Conventional injection molding does not lend itself to high shear mixing sufficient to achieve the sought after chemical reaction between components (b) and (c) and the required phase morphology.
Typical injection molding machines use a single screw which both reciprocates and rotates within a barrel in the following sequence of steps which constitute the molding cycle:
(i) screw forward or injection time PA1 (ii) hold time PA1 (iii) mold open time or boost. PA1 (1) bringing the molten components together essentially for the first time; PA1 (2) subjecting the molten components to shear on an intermittent basis; and PA1 (3) intermittently fabricating in one-step the sheared molten composition to articles of pre-determined shape.
During the screw forward time, the screw reciprocates (rams) towards the injection port (nozzle) of the machine to force molten resin into the mold. Also included in this step is the time the screw is held in the forward position to keep the mold full of molten resin as the molded article starts to solidify.
During the hold time, the screw rotates and retracts under the pressure of the molten resin being forced by the screw into the forward end of the barrel, i.e., adjacent to the injection port of the barrel. During this rotation, the resin feed to the injection molding machine becomes melted and transported into this injection position. Normally, when the screw retracts to a certain point, this means the forward end of the barrel is filled with the desired amount of molten resin and the screw stops rotating. Additional hold time is typically taken up with the screw positioned stationary in the retracted position until the molded article has cooled sufficiently.
During the mold opening step of the cycle, the screw remains stationary and retracted while the mold opens and the molded article is removed from the mold.
A typical molding cycle might take 55 seconds, consisting of 25 seconds screw forward time, 25 seconds hold time, and 5 seconds mold open time. Typically the screw rotates for only a portion of the 25 second hold time.
Because the screws typically used in injection molding machines are desired to merely transport and melt the thermoplastic material, and because the molding cycle typically includes only a small proportion of time when the screw is rotating, pre-compounding has served as the standard for polymer resin preparation, particularly where reaction of components is required and a resin component which is not a major volume component is forced into becoming a continuous phase. Heretofore, two step melt processing has been the only known method for producing the partially grafted thermoplastic resin compositions of Saltman I and II.